The invention relates to a highly chemically and thermally resistant borosilicate glass which is capable of forming a seal with tungsten.
Lamps operating at a temperature above 300.degree. C. are generally made of borosilicate glasses which have high thermal shock resistance. Due to their large processing range these glasses, which can only be softened at high temperatures, for instance at around 700.degree. C., are highly suitable for processing on pressing machines or with an open flame. A high service temperature is particularly desired in applications where the glass must be able to withstand very high ambient temperatures, for example in flashgun tubes or in oven windows.
Compared with normal soda-lime glasses, borosilicate glasses are also distinguished by great hardness, good surface quality, good chemical resistance and improved insulation capacity. Further, it is possible to make such glasses compatible with tungsten power supply wires passed through the glasses by matching the expansion behavior of the glass to that of tungsten.
Glasses of this type which have been known for years are listed, for example, in Pfander, SCHOTT Guide to Glass, van Nostrand Reinhold Company, New York 1983 (ISBN 0-442-27435-1), page 110. A more recent glass having the composition (in % by weight) 68-82 SiO.sub.2 ; 0.5-5 Al.sub.2 O.sub.3 ; 10-18 B.sub.2 O.sub.3 ; 3.5-8 Na.sub.2 O+K.sub.2 O+Li.sub.2 O; 0-3 CaO+MgO; 0.06-1 refining agent, is described, for example, in JP AS 92/33741.
However, increasing operating temperatures of lamps have resulted in a continual decrease in the safety margin of the glasses, in particular in lamps which are subjected to high temperatures and in the case of very long burning periods, so that it is desirable to further develop these glasses to give improved heat resistance.